Water-based inks and coatings are well known and many have been commercialized. Growth of such inks is expected to continue due to the economic and ecological factors. One such ink is disclosed in U.S. Pat. No. 4,172,064 to Keeler which comprises a four-component copolymer of an alkyl ester of acrylic acid, an alkyl ester of methacrylic acid, an unsaturated nitrile, an unsaturated carboxylic acid, a coalescing agent and a mixture of surfactants. The resulting copolymer has a Tg of less than 80.degree. C., amount of alkyl esters of acrylic and methacrylic acids can vary from 50 to 80% by weight of the copolymer, amount of the carboxylic acid can vary from 2 to 15%, and amount of the nitrile is greater than amount of the carboxylic acid. Difunctional and trifunctional inorganic salts can be used as curing agents and reactive crosslinking materials can be incorporated to improve physical properties of coatings or inks. Due to the presence of surfactants and unreacted carboxyl groups, such inks have poor scrub and water resistance. Furthermore, toxicity problems associated with the nitriles, such as acrylonitrile, are also a source of concern.
Another example of a water-based ink is a two-package ink in which the first package comprises a mixture of a binder resin, flatting agent, and a co-solvent, whereas the second package comprises a crosslinking agent. The binder resin has a molecular weight of about 25,000, and is a copolymer of less than 5 parts acrylic acid, about 65 parts methyl methacrylate, and about 35 parts ethyl acrylate which is prepared by solution polymerization in an organic solvent. The ink is made by mixing the two packages before use.
Although the two-package ink, described above, has a number of outstanding features, it has the self-evident disadvantage of having to mix two components before use and storage thereof before use. It is, therefore, desirable to overcome the disadvantage of a two-package system while retaining its desirable features.